In a gas turbine, hot gases of combustion flow from an annular array of combustors through a transition piece for flow along an annular hot gas path. Turbine stages are typically disposed along the hot gas path such that the hot gases of combustion flow from the transition piece through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The turbine buckets may be secured to a plurality of turbine wheels comprising the turbine rotor, with each turbine wheel being mounted to the rotor shaft for rotation therewith.
A turbine bucket generally includes an airfoil extending radially outwardly from a substantially planar platform and shank portion extending radially inwardly from the platform. The shank portion may include a dovetail or other means to secure the bucket to a turbine wheel of the turbine rotor. In general, during operation of a gas turbine, the hot gases of combustion flowing from the combustors are generally directed over and around the airfoil of the turbine bucket. Thus, to protect the part from high temperatures, the airfoil typically includes an airfoil cooling circuit configured to supply a cooling medium, such as air, to actively cool the airfoil's base material.
Conventionally, the external surfaces of buckets and nozzles of airfoils are cooled using a series of film holes defined through such surfaces. In particular, the film holes are typically drilled on the airfoil surface(s) and into the airfoil cooling circuit to permit the cooling medium flowing through the cooling circuit to be supplied to the airfoil surface. Similar film holes are also used to cool other turbine components (e.g., shrouds). However, it has been found that these film holes often provide for less than optimal cooling of turbine component surfaces. Specifically, since the film holes are drilled straight into the surface, the exit angle of the cooling medium expelled from the holes is relatively high, thereby negatively impacting flow attachment of the cooling medium against the surface. To address such flow attachment issues, various design modifications to the film holes have been proposed, such as by forming advanced-shaped film holes within the surface (e.g., chevron-shaped or bell-shaped holes) or by forming complex-shaped outlets for the film holes. However, many advanced-shaped film holes (e.g., chevron-shaped holes) are designed to spread coolant to the sides of the film hole which may result in non-uniform coolant distribution such as deficient coolant flow through the middle portion of the film hole. In addition, many advanced-shaped film holes such as chevron-shaped film holes create an internal medium flow vortex with a structure that provides insufficient cooling to particular portions of the airfoil.
Accordingly, a cooling arrangement that assists uniform coolant distribution, provides sufficient cooling through the middle portion of a film hole, and creates an internal medium flow vortex with an improved structure would be welcomed in the technology.